In the prior art, the American patent U.S. Pat. No. 6,043,574 is known, describing a two-phase motor formed by a stator part excited by electric coils and a magnetised rotor having N pairs of poles magnetised radially in alternate directions, N being equal to 3 or 5. The stator part has at least two W-shaped circuits each comprising an electric coil surrounding the central leg. The W-shaped circuits are disposed so that, when one of the central legs is situated facing a magnetic transition, the other central leg is situated facing a magnetic pole. The pole shoes of the legs of a W-shaped circuit being spaced apart angularly by π/4 and the pole shoes of the central legs of two W-shaped circuits belonging to different phases being separated angularly by an angle of approximately π/2±k·π/N, where N is the number of pairs of magnetic poles, that is to say 3 or 5, and k is equal to 0, 1 or 2.
The French patent FR 2807160 of the company DENSO is also known, describing a display measurement apparatus comprising a dial, a housing disposed behind the dial, a drive control section disposed in the housing, and a rotary shaft driven by the drive control section.
The rotary shaft is composed of two thin shafts supported respectively by two bearings and a thick shaft connecting the two thin shafts. The length and outside diameter of each thin shaft are such that the ratio of the length to the outside diameter guarantees sufficient bending strength. Consequently the rotary shaft turns without any jolts and a display needle carried by the shaft indicates precise values of an operating condition of the vehicle.
The technical objective of the solutions of the prior art is to increase the torque and to eliminate the flux closure piece without losing the advantage of the simplicity of manufacture and the possibilities of integrating associated mechanisms such as reducers.
The Japanese patent application JP 2004 251354 is also known, describing an actuator improved so as to remedy the problem of the flow of lubrication oil towards an output rotation shaft.
This patent describes an actuator for a measuring apparatus comprising a housing, a rotor rotated by interaction with a stator and a speed reduction gear train for decelerating the rotation of the rotor.
The speed reducer comprises a train of output reducing gears in which an output rotation shaft is placed at the centre of the part projecting out of housing and the teeth are disposed on the external periphery. In the output gear, the grooves are formed so as to prevent the lubricating oil from flowing from the part of the teeth of the output rotation shaft. The grooves are provided in the radius direction of the output gears around the output rotation shaft.
The device has arms that are intended to recover the oil, which is transmitted towards the seat in order to prevent the oil spreading over the display window.
These rubbing arms have an oil discharge function.
The actuator described European patent EP 1244200 is also known, comprising a stepping motor provided with a rotor having a plurality of magnetic poles along the circumferential direction thereof, a pair of magnetic yokes arranged to face the magnetic poles of the rotor, and a pair of excitation coils to excite the pair of magnetic yokes respectively in response to pulses that produced the phase differences.
The European patents EP 1458082 and EP 1450138 describing variants of actuators are also known.
These solutions of the prior art are roughly satisfactory, but it appears that the movement of the driven member exhibits, in some cases, jolts, resulting firstly from the functioning and characteristics of the motor (torque without excessively high current and torque with non-constant current) and secondly from the clearances in the gear train. These jolts are in particular present when certain speeds are reached by the display. From the point of view of the user, the first drawback is the lack of fluidity of the movement perceived by the user, which represents visual discomfort as well as loss of precision of information. The second drawback, due to the excessive vibrations produced by these jolts, is a noise audible to the user, also representing discomfort. Finally, these same vibrations may have a detrimental effect on the service life of the motorised assembly, requiring early replacement thereof.